For the operation of an elevator system, a reliable electric power supply system is required. Normally, the supply of electric power is obtained from traditional mains current and the electric energy obtained can be transformed to a desired voltage level by means of transformers. For faultless operation of the elevator system, uninterrupted supply of electric power is required, because the nature of the elevator system is such that a power failure may result in an elevator with passengers on board stopping between floors and thus in a danger situation. Therefore, elevator systems are provided with standby power sources for the supply of electric energy in cases of emergency.
Prior-art emergency power sources so far used in elevators are batteries and/or safety circuits. The batteries used have typically been lead acid batteries, which have a maximum service life of the order of five years. An example of a prior-art standby power source is presented in specification U.S. Pat. No. 4,316,097, which includes a battery and an associated circuit.
So-called supercapacitors are able to store a considerably larger electric charge than ordinary capacitors. Supercapacitors may have a capacitance in the range of e.g. 100 . . . 2000 F. Therefore they offer an interesting possibility for the storage and supply of electric energy. It is expected that supercapacitors will replace lead acid batteries in many present-day applications in the near future. At present, the biggest obstacle to this is the expensive price of supercapacitors.
A plurality of supercapacitors can be connected together to form a so-called supercapacitor pack, which has a greater electric charge storing capacity and provides a higher voltage than an individual supercapacitor. Supercapacitors are capacitors containing a double-layer structure, wherein the electrodes consist of active carbon. Consequently, the capacitor contains several thousands of square meters of so-called effective area per gram of carbon, and it contains two electrodes separated from each other by a very small distance, which is of the order of nanometers. Due to these properties, supercapacitors have a very large capacitance, which may be hundreds or even thousands of farads.
In the maintenance of elevators, material costs constitute a fairly small proportion in relation to other costs, because most of the maintenance costs consist of the salaries and travel costs of maintenance personnel. For this reason, supercapacitors provide an important alternative as a standby power source not-withstanding their high price.
A supercapacitor works like an ordinary capacitor, which is capable of storing electric charge. For this reason, they can not be connected directly in place of batteries to replace these. The voltage of a capacitor depends linearly on its electric charge, whereas the voltage of a lead acid battery is non-linear in relation to the charge. At high charge values, the battery voltage remains nearly constant, and as the charge diminishes, the voltage falls rapidly from the constant value to zero. From the foregoing it follows that most power source solutions containing supercapacitors comprise a circuit for stabilizing the supercapacitor's output voltage as an AC or DC supply voltage of a desired level.
According to a prior-art example, the voltage obtained from the terminals of a supercapacitor having an energy capacity of 10 Wh varies within the range of 20 V . . . 60 V. When the terminals are connected to a separate stabilizer (power requirement about 4 kW), the stabilizer will give either a 48-V direct voltage or a 230-V alternating voltage at its output.
As the voltage (so-called cell voltage) of one plate capacitor is low (typically of the order of 2.5 V), it is necessary to connect numerous capacitors in series to produce one supercapacitor. Besides the above-mentioned reasons, stabilizers are also needed because of the high price of the supercapacitor. Considered from a technical perspective, it is possible to connect even hundreds of capacitors in series. Large capacitor series like this are used e.g. in trains.
The price of a supercapacitor today is of the order of about 40 to 80 euros per watt-hour, depending on the voltage obtainable, and the energy obtained per unit mass is about 3.5 Wh/kg. The packing technology leads to high costs of high-voltage capacitor units.
Specification U.S. Pat. No. 6,742,630 describes the use of supercapacitors as an energy source, or rather as an energy storage, in an elevator system. During acceleration and braking of the elevator, a great deal of power is needed, and this is obtained partly from a power source consisting of a supercapacitor to the motor. Another objective is to balance the energy consumption via charging and discharging of a supercapacitor so that when the elevator (or elevators) is/are stationary, the supercapacitor is charged taking the energy from the main power source (mains power) and the additional energy required during accelerating movement of the elevator is taken from the charged supercapacitor instead of the mains. Furthermore, the supercapacitor can be used to store energy obtained during braking.
A reserve power solution commonly used in elevators is lead acid batteries, which involve the problem of a relatively short service life, typically about five years. In addition, they have a large size and give a fairly low voltage level, which is why it is necessary to connect a large number of them together, with the result that the battery system takes up an undesirably large space. The battery system has to be provided with power transformers to supply a three-phase alternating current to the elevator motors, and consequently the system is very complicated. For the elevator system to work reliably, an apparatus that is simple in operation and contains a small number of components is required.
Electric converters are basically reliable. However, elevator applications require a wide power range (5 . . . 100 kW), which is why it would be necessary to have several converters to produce different output powers. This further leads to quality problems due to the complicated nature of the apparatus, as well as high equipment costs.